I'm trying to connect an APC UPS SMT750I from its RJ45 console port to the CRS326-24G-2S+RM via its serial console RJ45 port. I used RJ45-to-RJ45 rollover cable to do so.
However the RouterOS is not recognising it. I have AP9630 NMC2, but as far as I read the SNMP is not supported by RouterOS as well.
Any idea how can I connect the APC UPS SMT750 to the CRS326-24G-2S+RM router? CRS326-24G-2S+RM doesn't have USB port. Perhaps from the UPS's USB-B port to the RJ45 console of the router? But I don't know the right cable wiring...
Some help is appreciated.

My suggestion: just don't. As you already discovered, UPS support in ROS is not worth mentioning, plus UPS control from router doesn't add any value (to uncontrolled UPS).

I'm trying to connect an APC UPS SMT750I from its RJ45 console port to the CRS326-24G-2S+RM via its serial console RJ45 port. I used RJ45-to-RJ45 rollover cable to do so.
However the RouterOS is not recognising it.

What makes you think a rollover cable will work? The APC connector is actually 10-pin, although the outer pins are only required for USB communications. You could make up a custom lead, the pinouts can be found on the internet https://pinoutguide.com/dev/APC/Smart-UPS+SMT750/, but note that the SMX/SMT/SURTD/SRT models and other next generation APC UPS's use Modbus RTU for communications, not the older UPS-Link / Smart Signalling protocol without the use of the AP9620 card.

As @mkx said the Mikrotik UPS support is not great. I've found the serial APC Smart Signalling support better than the USB support, and Mikrotik do not support Modbus RTU. The only real use is to monitor the UPS with a scheduled script, e.g. to send emails on power fail. With an NMC2 card fitted you might as well just use the email notifications in that.

My suggestion: just don't. As you already discovered, UPS support in ROS is not worth mentioning, plus UPS control from router doesn't add any value (to uncontrolled UPS).

Thanks for the suggestion. I actually wanted to do graceful shutdown in case the UPS shutdown due to low battery. Monitoring wasn't important as I can do it via the NMC. My concern was of a sudden shutdown that sometimes there could be loss of data or something goes corrupt, but with a backup is quickly fixable. Wasn't sure whether it's worth it.

What makes you think a rollover cable will work? The APC connector is actually 10-pin, although the outer pins are only required for USB communications. You could make up a custom lead, the pinouts can be found on the internet https://pinoutguide.com/dev/APC/Smart-UPS+SMT750/, but note that the SMX/SMT/SURTD/SRT models and other next generation APC UPS's use Modbus RTU for communications, not the older UPS-Link / Smart Signalling protocol without the use of the AP9620 card.

As @mkx said the Mikrotik UPS support is not great. I've found the serial APC Smart Signalling support better than the USB support, and Mikrotik do not support Modbus RTU. The only real use is to monitor the UPS with a scheduled script, e.g. to send emails on power fail. With an NMC2 card fitted you might as well just use the email notifications in that.

I thought rollover cable would work as the that's how you can connect the APC UPS form its serial port to another device with RJ45 serial port. And the port of this MikroTik Switch is a serial port.
Monitoring the UPS from the router is of no interest to me.
So basically the only chance to connect the UPS is with legacy AP9620 card? Cannot use it's USB-B port or the RJ45 serial one?

I thought rollover cable would work as the that's how you can connect the APC UPS form its serial port to another device with RJ45 serial port.

I don;t think I've ever seen an APC connection which didn't require a custom cable to swap pin connections as they are not a 1-to-1 mapping.

So basically the only chance to connect the UPS is with legacy AP9620 card? Cannot use it's USB-B port or the RJ45 serial one?

For serial it looks that way, APC have removed the older serial communications protocol and Mikrotik do not support the newer one. The USB signalling would probably work, but your Mikrotik doesn't support USB.

Even if you did manage to connect it the behaviour may not be what you expect. If the Mikrotik is configured to shutdown on low battery warning and that occurs BUT the mains is restored before the UPS powers off the Mikrotik will still be in shutdown, it requires a power cycle to restart it.

Even if you did manage to connect it the behaviour may not be what you expect. If the Mikrotik is configured to shutdown on low battery warning and that occurs BUT the mains is restored before the UPS powers off the Mikrotik will still be in shutdown, it requires a power cycle to restart it.

For this reason I'm using a trick, but I'm not sure if it's doable on MT: when UPS is in low-battery state, my linux box enters a reboot sequence ... the last command, issued in that reboot sequence, is order to shutdown UPS. My UPS (an Eaton unit) only honours that command if mains is not present. Hence if at this stage mains is still failed, everything will shut down and later, when mains returns, everything will power back again. If at that moment mains is already back, the linux box simply reboots.

Surely shutdown script is aware if shutdown is due to depleted UPS battery or if shutdown is an ordinary shutdown and in the later case it performs actual shutdown (in PC case this means shutting off PC while UPS remains working).

The USB signalling would probably work, but your Mikrotik doesn't support USB.

Would it work if I use a USB to RJ45 Cisco Console Cable with FTDI chip? From UPS USB-B port to the serial RJ45 of the MikroTik? Or somehow connecting the USB port of the UPS to the serial RJ45 port of the MikroTik?

Thanks @mkx - a good tip.

Would it work if I use a USB to RJ45 Cisco Console Cable with FTDI chip? From UPS USB-B port to the serial RJ45 of the MikroTik? Or somehow connecting the USB port of the UPS to the serial RJ45 port of the MikroTik?

No. USB only supports host-to-peripheral communications, not host-to-host or peripheral-to-peripheral, both the UPS and FTDI serial converters are USB peripherals.